1. Field of the Invention
This invention relates to a silicone composition which is useful as an impregnating agent for electronic parts and has good moisture proofing and good adhesiveness to adhesives.
2. Description of the Prior Art
It is the usual practice to encapsulate or package electronic parts, such as transistors, diodes, resistors, capacitors, ceramic-dusted capacitors and the like, with epoxy resin compositions. In general, there are formed very small gaps or voids between the encapsulating resin and the electronic parts being encapsulated.
The presence of the gaps or voids may lead to deterioration of the characteristics of the electronic parts because impurities such as water, Cl.sup.-, Na.sup.+ and the like which will cause the characteristics of the electronic parts to deteriorate can enter from the outside through the gaps or voids.
For the purpose of filling the gaps or voids, natural waxes such as carnauba wax or silicone oils have been conventionally used. Especially, silicone oil has been widely employed because the oil has such a low surface tension that it can readily enter small gaps, and contains only reduced amounts of impurities such as Cl.sup.-, Na.sup.+ and the like. In addition, the silicone oil has good heat resistance.
In recent years, attachment of electronic parts to a printed circuit board has been often made by the following procedure so as to improve the productivity there of. Electronic parts are preliminarily adhered to a printed circuit board with an adhesive, followed by immersion in a solder bath to fix the electronic parts onto the printed circuit board. When known silicone oils are employed, gaps or voids involved in electronic parts can be filled but a slight silicone film is formed on the surface of the parts. Consequently, the adhesion between the silicone film and the adhesive is poor, which results in the adhesive not adhering to the electornic parts, thus presenting the problem that the electronic parts cannot be mounted on the printed circuit board.
Since silicone impregnating agents or oils can fill the defective parts of electronic parts and can improve a moisture proofing characteristic, they have been frequently used to treat transistors or IC having metal leads. When, however, there are used as the silicone impregnating agent those of the curing type comprising, for example, organohydrogenpolysiloxanes and vinyl group-containing organopolysiloxanes for the treatment of electronic parts, a silicone film is formed on the surfaces of the electronic parts or on the metal leads thereof. The formation of the silicone film tends to adversely influence the printability on the surfaces of the electronic parts or the solderability of the metal lead to a substantial extent. Accordingly, for the soldering or printing operations of the electronic parts treated with such a silicone impregnating agent it has been necessary to remove the silicone film deposited on the lead portions or the surfaces of the electronic parts with a solvent or by blasting. This creates a complicated operation procedure for assembling electronic parts, which in turn places a limitation on the use or application of the electronic parts. To avoid this problem, a non-curing type of silicone impregnating agent has been used. If this type of agent is used, it is easy to remove the silicone deposits from the lead portions or the surfaces of electronic parts. However, when the impregnated electronic parts with the silicone are allowed to stand under high temperature and high humidity conditions over a long period of time, the silicone is gradually deluted, thus producing a problem of deterioration of the electric characteristics.
As will become apparent from the above, known impregnating agents are not satisfactory since they are disadvantageous in that the electronic parts treated therewith are poor in working properties or the agents are not high in moisture proofing. These agents are not favorable as an impregnating agent for recent electronic parts. Accordingly, there is s demand of the development of a novel impregnating agent.